This invention relates to communications systems and more particularly to communications between an airborne or spaceborne vehicle and an underwater receiver.
Submarines complete large portions of their missions while being submerged. Occasionally, while the submarine is submerged an airborne vehicle may want to communicate with the submarine. The prior art utilized two methods for the transmission of information from an aircraft or satellite to points under water. The first method used electromagnetic energy transmitted from the aircraft or satellite to carry the signal. Electromagnetic (EM) energy does not propagate well in water, except in certain bands of the EM spectrum. Usable bands of the spectrum are the Extremely Low Frequency (ELF), Very Low Frequency (VLF) and the Visible Light bands (in the blue-green regime). The disadvantages of the foregoing method are that both VLF, ELF and blue-green signals are difficult to generate and transmit without heavy and massive equipment. The VLF and ELF communication schemes employ a very long and cumbersome antenna which must be deployed from the aircraft or satellite, and a similar antenna for receiving the electromagnetic signals must be deployed under water. An aircraft or submarine""s agility is degraded by the deployment of such an antenna. The blue-green light communication scheme is very inefficient. Therefore, a very powerful laser must be used to transmit coherent blue-green light. The underwater receiver is a complex and highly sensitive light detector which employs very narrowband atomic transitions. The signal to noise ratio of the receiver at the receiving point is very low, since most of the light is scattered and attenuated as it propagates down from the water surface. Both low frequency electromagnetic techniques and blue-green communication require the receiver to be within at least 1,000 ft. of the water surface, which is not always practical. The second method used by the prior art for the transmission of information from an aircraft or satellite to an underwater receiver utilized the transmission of an RF signal to a surface ship or buoy. The buoy or surface ship then retransmitted the message underwater using acoustic energy. Acoustic energy in the sonic frequency regime can propagate miles underwater, thus making this scheme advantageous. However, some disadvantages of the foregoing method are that if the receiver is moving (e.g., if communication is to a moving submarine) the receiver may still move out of range of the acoustic transmitter, requiring the surface ship to move or deploy new expendable buoys. Furthermore, at distances of several miles from the acoustic transmitter, the transmitted information may arrive by several propagation paths, which are caused by refraction of acoustic energy by thermal gradients and reflections of the acoustic signal from the water surface and the ocean""s bottom. This xe2x80x9cmultipathxe2x80x9d phenomenon is similar to reverberation in a room of bad acoustic design, and can result in reduced intelligibility of the communication. It was also possible for an unfriendly power to intercept or jam the prior art methods which relied on ELF, VLF or RF transmission.
This invention overcomes the disadvantages of the prior art by utilizing the direct conversion of EM or particle kinetic energy into acoustic energy. The foregoing is accomplished by using either a pulsed infrared wavelength laser or particle beam which is fired into the water from an aircraft or satellite. The physical mechanisms producing sound are of two kinds: (1) thermal expansion of the water from heat generated by medium attenuation of a pulse of laser light or impinging particles, or (2) explosive vaporization of a small volume of water when the heat deposited by the laser or particle beam is large enough to raise the local water temperature above boiling threshold. Infrared laser light is usually used because of its high attenuation coefficient in water, which causes high thermal densities. The level of sound produced by infrared lasers is sufficient for communications at expected ranges of communication buoys. Infrared lasers may be controlled (modulated) to the extent required for an underwater communications system. Typical data rates are xcx9c1-10 bits per second.
Modulation schemes which may be employed are on-off keying (OOK), pulse duration modulation (PDM), pulse amplitude modulation (PAM), and frequency shift keying (FSK). The foregoing modulation schemes may be used for lasers and particle beams.
When the density of the heat energy deposited by laser beam absorption is less than that required to vaporize a local volume of water (xcx9c2500 joules/cm3) the acoustic pressure at radial distance R and polar angle xcex8 from the beam impact point at the water surface is given by the following expression:       P    ⁡          (              R        ,        t        ,        θ            )        =            k              2        ⁢                  xe2x80x83                ⁢        π              ⁢                  ∫                  -          ∞                          +          ∞                    ⁢              xe2x80x83            ⁢                        ⅆ          ω                ⁢                  xe2x80x83                ⁢                  M          ⁡                      (            ω            )                          ⁢                  ω          2                ⁢                              exp            ⁡                          [                                                -                  j                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            ω                      ⁢                                              xe2x80x83                                            ⁢                      t                                        -                                          R                      /                                              c                        o                                                                              )                                            ]                                ·          sin                ⁢                  xe2x80x83                ⁢        θ            
where k=xcex2Io/(4xcfx80RcoCp)
Co=speed of sound
Cp=specific heat of water
Io=laser power output
t=time
xcex2=thermal expansion coefficient of water
Here M(xcfx89) is the Fourier transform of the modulation, and Io the laser power output prior to modulation. The above expression assumes that the useful portion of the acoustic signal is transmitted at a frequency with wavelength smaller than either the beam spot size or absorption depth.
If the modulation is a gaussian pulse       M    ⁡          (      t      )        =                    M        o                              2          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢                      σ            t                                ⁢          exp      ⁡              [                              -                          t              2                                /                      σ            t            2                          ]            
where "sgr"t≅(one-half of the laser pulse width). The Fourier transform of P(R,xcex8) is proportional to the function F(xcfx89)=xcfx892 exp[xe2x88x92xcfx892"sgr"t2].
The frequency (xcfx89xcfx81) when the spectral energy is the acoustic pulse peak is       ω    p    =                    1                  3                    ⁢              σ        t              -    1  
as can be found by setting the derivative of F(xcfx89) equal to zero.
Thus, the duration of the laser pulse (2"sgr"t) controls the spectral Wp. The bandwidth of the signal can be controlled by firing the laser a number of times at a repetition interval less than or equal to the duration of an acoustic pulse produced by a single laser pulse, or by simply lengthening the pulse duration for a single pulse. The pulse amplitude may be controlled and varied by changing the laser power output.
The extremely short 1-10xcexc absorption length (xcex4) for certain infrared light frequencies in water makes an explosive vaporization mode of thermoacoustic generation attractive. Incident light with a fluence of  greater than 3 J/cm2(Er) at 10xcexc wavelength, for instance, will instantaneously boil the 10 micron layer in which most of the light is absorbed. This rapid vaporization produces an explosive stress or shock wave (with Fourier transform S(xcfx89)) which eventually propagates through the water as a soundwave (with Fourier transform proportional to xcfx89S(xcfx89)). The internal energy (E) contained in the gas that was vaporized is approximately given by the ideal gas state equation:
E=3/2 P V
where E is the difference between the laser energy and the threshold energy required to boil the thin layer of water. The initial pressure in the gas bubble would approximately be given by       P    o    =            2      3        ⁢          xe2x80x83        ⁢                  (                              E            o                    -                      E            T                          )            V      
where:
Eo=laser pulse energy
ET=Threshold for vaporization
V=Volume of fluid in which absorption of light occurs
V=Axcex4=(spot area)xc3x97(laser light absorption depth)
Reasonable values for the spot area (A) and absorption lengths are:
A=spot area=1 CM2=10xe2x88x924m2 
xcex4=absorption length of fluid=10xe2x88x925m at CO2 laser wavelengths
The determination of allowable communication path length requires a knowledge of the spectral level and distribution of the acoustic energy represented by the source strength given above. The duration of the time domain pulse resulting from explosive vaporization of the water surface layer must be estimated to obtain its spectral distribution. Assume the laser pulse is sufficiently short ( less than 10xe2x88x926 sec.) so that all the laser energy is absorbed before the explosive vaporization has appreciably progressed. The time required to expand 10xe2x88x929m3 volume of water to 1 ATM gaseous phase is roughly one-half the width of the acoustic pulse produced. The expanded volume of the water is 10xe2x88x926m3 based on the roughly 103 difference in density between liquid water and water vapor at 1 ATM. The vapor bubble expands at roughly Mach two in air (2200 m/sec.) forming a spherical segment of volume xcx9c10xe2x88x926m3.
The time for the expansion to take place is
T=4.5xcexc sec.
at Mach two. The center frequency of the wideband pulse thus produced is
fo=(1/(9 xcexcsec))≅110 KHz
The spectrum of the thermoacoustic pulse is a roughly 100% bandwidth pulse centered on fo thus with single pulse on-off coding the signal bandwidth is (BW)=110 KHz.
Taking, for example, a 10 joule laser pulse, the peak pressure at the surface is             P      s        =                  2        3            ⁢              xe2x80x83            ⁢                        (                                    E              o                        -                          E              r                                )                          A          ⁢                      xe2x80x83                    ⁢          δ                                P      s        =                  2        3            ⁢              xe2x80x83            ⁢                        (                      10            -            3                    )                                      (                          10                              -                4                                      )                    ⁢                      (                          10                              -                5                                      )                              
or
293 dB relative to 1 xcexcPa (relxcexcPa)
Assuming spherical spreading from an initial radius (Ro) of the source, the source strength at a range R is       P    ⁢          (      R      )        =            P      o        ⁢                  (                  R          o                )            R        ⁢          f      ⁡              (                  θ          ^                )            
where xcex8 is the horizontal propagation angle, and f(xcex8) is the source directivity (≈sinxcex8). The initial radius can be taken as V⅓ where V=10xe2x88x926m3 so that Ro=10xe2x88x922m. The resulting source strength at 1 meter below the beam impact point (sinxcex8=1) is then
SL=20 log P(1)=293xe2x88x9220 log 104 or
SL=213 dB re (lxcexcPa)
The standard sonar equation can be used to estimate the excess signal at a distance r meters from the source. In the above example, the spectrum of the acoustic signal is approximately linear with frequency for xcfx89 less than xcfx89xcfx81. Thus, the spectra level (dB//Hz) at 10 KHz (our assumed transmit frequency) is dB below that at 110 KHz. The spectrum level for   θ  =            π      2        ⁢          xe2x80x83        ⁢          rad      .      
a 110 KHz is about 213 dBxe2x88x9210 log (1.1xc3x97105 Hz)≅163 dB//Hz. Therefore, the acoustic spectrum level at 10 KHz≈143 dB//Hz The sonar equation is inverted to give Figure of Merit (maxim propagation loss) for good communication reliability. This yields (Figure of Merit) FOM=143xe2x88x9245xe2x88x9212 =86 dB=source spectrum levelxe2x80x94noise spectrum levelxe2x80x94threshold.
The signal-to-noise ratio required to reliably communicate is assumed to be 12 dB. The range of the signal pulse on-off keyed communication system described above corresponding to an 86 dB FOM is 6 Kyd. Receiving the signal with a directional receiver will increase this range considerably. A practical system calls for bit rates of the order of 5 bits a second or 50 watts of laser power with 10 joule pulses.
An alternate use of the laser energy would be to fire the laser every xcfx84 sec to obtain a more narrowband acoustic wave train centered on xcfx84xe2x88x921. For instance, a ten cycle burst at the same laser power per pulse (10 J) cited above would require 100 joules. The bandwidth would be 11,000 Hz. Thus, if coherent detection could be used, an extra 10 dB of transmission loss could be tolerated.
A particle beam generates acoustic energy by impacting a small region of the surface of the water at the air/water interface. Energy from the aforementioned beam is absorbed by the water which causes the water to be heated. The heating of the water causes thermal expansion which generates pressure or stresses within the water that propagate through the water as a sound wave. The pressure P produced by the particle beam is given by expressions provided above for the thermoelastic energy case with the power flow in the particle beam replacing the laser power in the formulas.
Thus, by turning the particle beam on and off, a code similar to the one hereinbefore described may be produced because different amounts of energy will be absorbed by the water at different intervals of time causing acoustic signals to be produced which may be received by a sound detector.
It is an object of this invention to provide a new and improved communications system between an airborne or spaceborne vehicle and an object that is underwater.
Other objects and advantages of this invention will become more apparent as the following description proceeds, which description should be considered together with the accompanying drawings.